Is It Hard To Change Mower Gasket?

Head gasket failure is common especially in hard working older mowers. I’m a mechanic for over twenty years and I’ve replaced a ton of them. Mowers aren’t complex, most repairs including gasket replacement is possible with just a little knowledge, and this post covers it all.

A mower head gasket is not difficult to replace. However you will require a torque wrench to tighten the cylinder head to the correct specification, this procedure is crucial to successful head gasket replacement.

To replace a mower head gasket you’ll need to remove some components, they include:

  • Pull assembly
  • Carburetor
  • Muffler
  • Valve cover

In this post you’ll learn how easy it is to replace your mower head gasket and how to use a torque wrench.

Before working on your mower, always remove the plug wire to prevent accidental starting.

Mower pull assembly

Pull assembly & Housing

Remove Mower Pull Assembly

Begin by removing the pull assembly. We need to remove the pull assembly as it allows us access to the cylinder head. The pull assembly is as its name suggests the assembly that houses the pull cord and recoil.

Some mowers will use a one piece assembly others will employ a two piece. Several fasteners (four usually) removes the assembly.

Mower carburetor

carb

Remove Mower Carburetor

Begin removing the carburetor by first removing the air filter. You’ll also need to clamp the gas line or drain down the gas tank.

With the air filter removed, and the gas line clamped, go ahead and remove the fuel line.

A couple of fasteners hold the carburetor to the engine, when you remove them the carburetor will be free. Note where the carburetor gaskets are positioned and orientated, replacing these incorrectly may cause the carburetor to make an ineffective seal and result in a surging engine.

With the gaskets noted, unhook the throttle springs by turning the carburetor sideways.

Set the carburetor aside and store carefully.

Mower muffler

muffler

Remove Mower Muffler

Some mowers may not require the muffler replaced and if you can access the cylinder head bolts, feel free to leave the muffler in place.

If on the other hand you need to remove it, use caution, muffler bolts often break when removing. If you can, spray some WD40 to help reduce the risk of breaking.

Two bolts are common and the bolts may employ a locking tab.

Mower valve cover

Valve cover

Remove Mower Valve Cover

The valve cover lives at the front of the engine and usually has OHV (Over Head Valve) embossed. To remove the cover you’ll need to remove four fasteners.

Best to elevate the front of the mower engine, this helps prevent old flowing forward and out onto the mower deck. Have a rag handy as there’ll be some oil spill.

Set the valve cover and fasteners aside. Note also the valve cover gasket may be damaged in the removal process, that’s just the way it goes.

If that is the case you’ll need to replace the gasket or use a heat and oil resistant silicone gasket maker.

Mower head bolts

head Bolts

Remove Mower Head Bolts

Pushrods which are metal rods used to push the rockers will need to be removed also. The intake and exhaust rods may be different, note them before removing the removal.

With all the bolts removed, you are now free to remove the cylinder head. You may need to loosen it by tapping it with a piece of timber, avoid prying with a lever or tapping with a hammer.

Mower cylinder head Mower cylinder head

remove Pushrods

To remove the head the head bolts must be loosened in a star sequence and initially each bolt loosened by a quarter turn before moving to its opposing bolt. This prevents warping the cylinder head.

The bolts (approx. six) are located on the cylinder head, some may be located behind the valve cover.

Remove the head and set it aside carefully, alloy is a soft metal and will damage easily.

Mower cylinder head

remove the head

Cleaning Mower Head & Block

To prepare for the new gasket, both surfaces will need to be cleaned. Old gasket material tends to stick to the surfaces.

To clean them you’ll need to use a plastic scraper, avoid metal scraper or wire wheel. Metal tools will damage the machined surface and could cause premature gasket failure.

Cleaning can be tedious but a good scraper and some white spirits will clean it up in jug time.

Mower cylinder head

Use Plastic scraper

Using Torque Wrench To Tighten Mower Head

Fitting the new gasket is easy, most will only fit one way. Go ahead and fit the gasket to the engine, two dowels are employed to help locate the gasket and also the head.

Before fitting the cylinder head, clean the bolts using a wire brush but don’t oil them. Lubing the bolts interferes with the torqueing measurements.

Mate the cylinder head with the block, be sure it seats correctly on the block dowels. Hand thread all the bolts into the head until they seat.

At this point we’ll need a torque wrench to nail the procedure correctly.

It is not advisable to tighten without a torque wrench. Getting this part wrong risks warping the head and damaging the gasket.

Torqueing mower cylinder head bolts

new head gasket

To use a torque wrench you’ll first need the cylinder head specs. Every engine make is different, so you’ll need to check yours.

Torque specs are measured in ft. lbs. (Feet Pounds) USA and Nm (Newton Meters) Europe. Your torque wrench will have both measurements.

To set the wrench, first loosen the lock then turn the handle to the desired measurement and tighten the lock again.

The torque wrench is now set to a specific torque setting and will give an audible click when it reaches that measurement.

If you need video help on setting a torque wrench, check out the video library or I wrote a complete guide on buying and using a torque wrench, check out here “Best torque wrench for mower blades”.

Ensure all bolts are tightened by hand until seated, now using torque wrench and socket tighten each bolt in a star pattern. Star pattern simply means tighten bolts in an opposing sequence (avoid tightening adjacent bolts sequentially).

Torque mower head bolts

Torque mower head bolts

When tightened, the pushrods will need to be replaced, be sure you are fitting the correct rod. Insert the rod into the block until it seats against the camshaft, now depress the rocker and valve and slide the rocker onto the rod tip. Repeat for the second rod.

Ideally valve clearance should be checked, it’s covered in the video library or check out this post “Mower engine valve adjustment”.

Reassemble now in reverse order, remember to replace the valve cover gasket, check and top up oil if needed.

Job done, that wasn’t so bad!

Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.